Photolithographic techniques used in the fabrication of semiconductor-based electronic devices rely upon a material that undergoes a chemical change when exposed to activating radiation. The material, known as a “photoresist,” is positioned over a semiconductor substrate and forms a mask over the substrate. Semiconductor substrates typically comprise wafers that have been fabricated from materials such as silicon, aluminum, polymeric resins, silicon dioxide, doped silicon dioxide, silicon resins, gallium arsenide, silicon nitride, copper, aluminum-copper mixtures and ceramics.
In semiconductor circuit fabrication, the photoresist mask acts as a template that imparts features of a microcircuit to the substrate. The photoresist mask is applied to the substrate by a method such as dip coating, spray coating, and spin coating. Mask thickness is adjusted in the spin coating method by adjusting solids content of the mask material.
In addition to the photoresist, photolithographic techniques utilize a polymeric composition in conjunction with a developer solvent that selectively removes only exposed portions of the photoresist in one embodiment, which is a positive acting photoresist, and only unexposed portions of the photoresist in another embodiment, which is a negative acting photoresist. This selective removal produces a patterned photoresist layer. The patterned photoresist layer provides a patterned mask for subsequent steps of circuit fabrication such as ion implantation, etching, or patterned deposition of materials by lift-off techniques. The lift-off techniques include depositing a microcircuit material all over the surface of the substrate, applying the patterned photoresist mask, treating the mask with the activating radiation, and then removing portions of the photoresist with the developer solvent in order to transfer the mask pattern to the microcircuit material. The steps of mask material application—mask formation and lift-off—may be repeated in order to vertically fabricate the microcircuit from the semiconductor and other materials.
Because critical dimensions of a semiconductor microcircuit are predetermined by dimensions of openings in the photoresist curing mask, it is essential that each step in the photolithography process transfer an accurately patterned mask for each subsequent step. It is important that critical dimensions are maintained throughout the photolithography process. Achieving the precise transfer requires that any surface overlaid by a photoresist be substantially free of discontinuities.
Maintaining critical dimensions becomes increasingly difficult to achieve as multiple layers are fabricated using the steps of coating a resist on a substrate to make a mask, forming a resist pattern on the mask, etching by using the resist pattern of the mask and stripping the resist to form a portion of a microcircuit. Any imperfection such as a surface discontinuity occurring as a result of fabrication of one of the layers becomes magnified in the fabrication of subsequent layers.
These imperfections such as surface discontinuities are manifested as a profile distortion at an interface between the resist and the substrate. The profile distortion takes the form of “footing” on positive resists and “undercutting” on negative resists. The problems of “footing” and “undercutting” prevent satisfactory control of patterning in the fabrication of critical dimensions using the photolithography technology. Thus, the usefulness of photoresist-based fabrication is significantly hampered, particularly for fabrication of tiny components.
This imperfection problem is particularly aggravated for acid catalyzed photoresists. The Knight et al. patent, U.S. number 5,486,267, which issued Jan. 23, 1996, describes a method for preparing an acid catalyzed substrate prior to applying a photoresist to the substrate. The method treats a substrate comprising chemically vapor deposited films of materials such as silicon dioxide and silicon nitride with a reactive oxygen species such as ozone.
The Katayama, et al. U.S. Pat. No. 5,372,677, which issued Dec. 13, 1994, describes another method for reducing profile distortion occurring as a result of an act of stripping an upper layer side resist. The method includes a step of treating a surface of a resist pattern with a plasma. The plasma is an oxygen plasma.
The Lind patent, U.S. Pat. No. 5,304,453, which issued Apr. 19, 1994, describes a method of forming a pattern on a semiconductor substrate. The method includes exposing a photo image, etch resistant pattern to an oxygen plasma. The pattern is transferred from its initial carrier substrate, the mask, to a receiver substrate by use of a hardenable liquid adhesive. An oxygen plasma is used to etch and to remove substrate material that does not improve adhesion.
The Pavelchek et al., U.S. Pat. No. 5,366,852, which issued Nov. 22, 1994, describes transferring patterns of micron and sub-micron dimensions to a substrate by dry etching. The dry etching method is set to utilize plasma or reactive ion etching to remove specific areas of material on a surface so that a pattern remains on the surface. The patent describes an embodiment where an organic polymer is used as a substrate and dry etching is performed using an oxygen plasma or oxygen reactive ion etchant.
The Putpuntambekar et al. patent, U.S. Pat. No. 5,714,037, which issued Feb. 3, 1998, describes several methods for improving adhesion between various materials used in fabricating integrated circuits. One of the methods includes improving adhesion between a silicon-nitride layer and a polyimide layer by treating the silicon-nitride with oxygen plasma.
A Japanese patent application number 59-070464, filed Apr. 9, 1984, to Sanyo Electric Company describes treating a chrome film with an oxygen plasma to roughen the surface of the chrome film. The roughened surface is overlaid with a hexamethyl disilazane.